Within the electronics industry, surface-mounted components, which are so named because they have conductive members adapted for soldering to metallized areas on the surface of a circuit board, are now becoming the components of choice. Such surface-mounted components are smaller in size than corresponding "through-hole" components, so named because they have leads designed for soldering to metal-plated through-holes in the circuit board. The reduced size of the surface-mounted components, as compared to their through-hole counterparts, permits a greater number of devices to be placed on a given-sized circuit board, allowing for increased functionality.
Presently, surface-mounted components are solder bonded to a circuit board by the following process. First, a layer of solder paste is applied to the metallized areas on the major surface(s) of the circuit board to which the conductive members of each surface-mounted component are to be soldered. Next, the surface-mounted components are placed on the circuit board so that each component has each of its conductive members in contact with a corresponding solder paste-coated metallized area. The paste is generally tacky and serves to hold the conductive members of the component to the metallized areas on the board. After the paste is applied and the components have been placed, the paste is rendered molten, that is, reflowed, typically by heating the circuit board in an oven designed for that purpose.
Most of the solder pastes now in commercial use contain rosin and other organic constituents which act as flux to wet the metallized areas during soldering. Invariably, when the solder paste is reflowed, the rosin and other organic constituents in the paste leave a residue on the circuit board. Depending on the type of solder paste used, these residues can be detrimental to the operation of the board. Moreover, the residues do adversely impact the ability to test the board and detract from its overall appearance as well. For these reasons, many manufacturers clean each circuit board after soldering to remove whatever residues remain.
Typically, the only way to clean the residues remaining on each circuit board after soldering is to use a very aggressive de-fluxing solvent, such as a detergent, a chlorinated solvent, or a chlorofluorocarbon (CFC). Spent detergents usually require treatment before discharge into a municipal sewer system, which raises production costs, while chlorinated solvents and CFC's have been shown to be detrimental to the environment. More recently, a terpene-based de-fluxing solvent has been developed which may be used to dissolve rosin-based residues on a circuit board. While many of the problems incurred with using detergents, chlorinated solvents and CFC's are eliminated by using a terpene-based de-fluxing solvent, there are other problems associated with use of such solvents to clean circuit boards containing surface-mounted components.
Residues also tend to remain on circuit boards containing through-hole components after the components are wave soldered to the board because rosin-based fluxes are commonly applied to the components prior to soldering. One proposal to reduce the residues remaining on circuit boards following wave soldering is to flux the component leads with a low solids flux which contains a reduced volume of rosin and organic constituents. After the components have been fluxed with the low solids flux, the components are wave soldered to the board in an inert atmosphere (typically nitrogen) within which a reducing agent (i.e., hydrogen, or an acid) is added to enhance the action of the flux. The combination of the inert atmosphere and the reducing agent has been alleged to greatly reduce the residues remaining after soldering. However, wave soldering is generally not suited for soldering surface-mounted components which have a large number of leads.
Thus, there is a need for a technique for soldering surface-mounted components to a circuit board so as to achieve reduced rosin residues on the board.